The present invention relates to casting silicon carbide parts. More specifically, the present invention relates to a method and apparatus for casting coarse grain siliconized silicon carbide parts.
Silicon carbide materials are often used in castings to design complex shaped parts that can withstand extreme conditions, such as those found in the semi-conductor, mineral processing, ceramic processing or oil industries. Moreover, it is generally preferred to siliconize a silicon carbide part during the firing process to further enhance the strength of the bonds in the material of the finished product. Siliconized silicon carbide castings are more often preferred over silicon carbide parts due to their increased ability to withstand highly abrasive or harsh chemical environments.
An exemplary casting process for a siliconized silicon carbide part starts with a formulated pre-casting mix typically composed of fine grained silicon carbide powders, distilled water, and dispersing aids or other chemicals. The pre-casting mix is placed into a mill to be tumbled and thoroughly mixed to produce an aqueous slurry. The slurry is then poured into a plaster mold, which removes most of the water and leaves behind a near net shaped green part. The green part is dried and then siliconized at high temperature to densify and strengthen the part. The part is then cooled and may be subjected to a finishing process, such as machining or polishing, to produce the final product.
However, under prior art manufacturing methods, the silicon carbide powders are very fine and present an overall surface area that is capable of adsorbing a lot of water. That is, prior art silicon carbide mixes typically have a maximum particle size of about 250-300 microns, and the particle sizes vary in distribution down to the sub-micron level. In total, the combined particles present a large surface area with which to adsorb water, and therefore require a water content of about 11% to 15% to form a suitable slurry.
Problematically, the more water needed to form a slurry, the more water that must be removed in order to form a solid part in its green state. For cast products with large cross-sectional thicknesses, this process can take time and effort, which can severely slow production rates of the final product. For example, prior art cast siliconized silicon carbide parts having cross-sectional thicknesses of 0.75 inches or more, may require upwards of 24 hours to remove enough water to form a solid green part.
Additionally, the silicon carbide particles must adequately fill up the spaces left behind by the water, or cracks and voids can develop in the finished part. As such the current practical limit of cross-sectional thickness of a siliconized silicon carbide cast part that can be manufactured using prior art methods is about one inch or less.
Unfortunately, the demand for larger and more complex cast siliconized silicon carbide products has outgrown prior art manufacturing capabilities. There is a growing need for cast parts having cross sectional thicknesses greater than 1 inch, and in some cases cross-sectional thicknesses of 8 inches or more are required.
In order to meet the demand for complex shaped parts having larger cross sectional thichnesses, prior art attempts at reducing the water content or increasing the grain size of the silicon carbide in a pre-casting mix have been made. However, formulating a coarser grained pre-casting mix and developing a method of casting the mix has proven to be difficult to do since several factors will affect the final product. Some of these factors include, but are not limited to, the proper combination of grain sizes and distribution, the proper material purity levels, the correct water content and other chemicals used in the mix. Moreover, the adjustment of one factor often has an effect on the other factors to further complicate the problem. Accordingly, finished prior art siliconized silicon carbide cast products with cross sectional widths of 1 inch or greater have yet to be produced consistently without developing an unacceptable number of flaws such as cracks or voids.
Based on the foregoing, it is the general object of the present invention to provide a formulated pre-casting mix and method of producing a siliconized silicon carbide cast product that overcomes the problems and drawbacks associated with the prior art.
The present invention offers advantages and alternatives over the prior art by providing a pre-casting mix, and a method of efficiently casting siliconized silicon carbide parts from the pre-casting mix, such that cross sectional thicknesses of the cast parts may be greater than 1 inch. The finished cast parts can be manufactured in complex shapes with thicknesses of upwards of 8 inches without an unacceptable number of flaws. Additionally, for most parts having cross sectional thicknesses of approximately xc2xe inches to 2 inches, the amount of time required to remove the water from the mold to form a solid green part is no more than 4 hours, as compared to 24 hours or more for prior art methods.
These and other advantages are accomplished in an exemplary embodiment of the invention by providing a method of casting a siliconized silicon carbide part. The method includes providing a pre-casting mix comprised of a silicon carbide powder having a maximum particle size within a range of about 1000 to 2000 microns, and water having a percentage by weight of no more than about 9.5% of the mix. A slurry is formed from the mix and poured into a mold. The water is then removed from the mold within a predetermined period of time to form a solid porous green part. The green part is surrounded with silicon metal, and heated to a predetermined temperature above the melting point of the silicon metal. The silicon metal then siliconizes the green part to form a siliconized silicon carbide part.
In an alternative embodiment of the invention, the method of casting also includes surrounding the green part with carbon. The green part, silicon metal and carbon are then heated to a predetermined temperature above the melting point of silcon metal. At the elevated temperatures, the green part, silicon metal and carbon are reaction bonded together to form a reaction bonded silicon carbide part.
Alternatively, the pre-casting mix may contain the carbon powder. Then the green part (with the carbon powder integrally include therein) and silicon metal are reaction bonded together to form a reaction bonded silicon carbide part.